It is known to produce relatively large electromagnets of superconducting wire for use, for example in magnetic resonance imaging (MRI) systems. Known magnets for MRI systems may be 2 m in diameter, 1.5 m in length and include many tens of kilometers of wire. Commonly, the magnets are composed of several relatively short coils, spaced axially along the axis of a cylindrical magnet, although several other designs are known, and the present disclosure is not limited to any particular magnet design.
Such superconducting magnets are not normally wound from a single length of superconducting wire. If several separate coils are used, they are usually produced separately and electrically joined together during assembly of the magnet. Even within a single coil, it is often necessary to join several lengths of wire together.
Joints between superconducting wires are difficult to make. Optimally, the joint itself will be superconducting—that is, having a zero resistance when the magnet is in operation. This is often compromised, and “superconducting” joints are often accepted which have a small resistance.
A common known manner of making a superconducting joint is to take the lengths of superconducting wire, and strip any outer cladding, typically copper, from the superconducting filaments from a length at or near their ends.
The superconducting filaments of the two wires may then be twisted together. The resulting twist of filaments is then coiled into a joint cup: a fairly shallow vessel, typically of copper or aluminum.
Alternatively, the filaments may be plaited, rather than twisted, before being coiled into the joint cup.
In other arrangements, the filaments of the wires are simply laid side by side, not necessarily touching one another, and placed within the joint cup. The superconducting joint is then made as described below.
The joint cup is then filled with a superconducting material, typically liquid Wood's metal, which cools and solidifies to embed the filaments in a superconductive mass. A typical joint cup may be a cylindrical vessel, closed at one end. FIG. 1 shows a conventional joint cup 10 into which wires 12 are introduced with their superconducting filaments 14 twisted together. In FIG. 1, the filaments are neither twisted nor plaited together. The joint cup is typically filled with a liquid superconducting joint material 28, such as molten Wood's metal. The superconducting joint material is then allowed, or caused, to solidify.
The present disclosure does not seek to change any of these features or method steps, but relates essentially to the joint cup itself.
Conventionally, superconducting magnets have been cooled by partial immersion in a bath of liquid cryogen, typically helium. This maintains the coils at a temperature below their superconducting transition temperature. By immersing the superconducting joints within the liquid cryogen, they can also be maintained below the superconducting transition temperature.
However, recent designs of magnets have avoided the cryogen bath, as being costly and in some circumstances wasteful of cryogen. These designs may be provided with a cooling loop or thermosiphon: a thermally conductive tube in thermal contact with the magnet which carries a circulating cryogen. The circulating cryogen is cooled and then introduced into the tube where it extracts heat from the magnet. The cryogen then expands or boils and circulates by thermal convection back to a reservoir where it is re-cooled. Circulation may be gravity induced or be assisted by any suitable means, such as a pump. A much smaller volume of cryogen is required than in an arrangement employing a cryogen bath. Cooling of the magnet coils is by conduction, through the wall of the tube, and possibly through the material of a structure supporting the magnet coils, such as a former.
In these cases, cooling of the joints is less effective than the more conventional immersion in liquid cryogen.
The present disclosure accordingly seeks improved superconducting joints and methods for cooling superconducting joints to enable the superconducting joints to be sufficiently cooled in magnets which are not cooled by immersion in a liquid cryogen.
In order to manufacture low cryogen inventory superconducting magnets—that is, those which do not rely on cooling by immersion in a bath of cryogen, but are cooled by a reduced volume of cryogen, for example in a thermosiphon or cooling loop—it is necessary to produce suitably cooled superconducting joints which do not require cooling by immersion in cryogen.
One approach to this problem may be in using flexible thermal conductors such as copper or aluminum braids or laminates thermally linking joints to a refrigerator, or by attaching superconducting joints to a cooled component using an electrically isolating adhesive layer. This latter approach is described, for example, in GB 2453734 (equivalent to US 2009/0101325 A1).
A difficulty with this latter option arises in achieving sufficient electrical isolation while maintaining adequate thermal conduction for effective cooling of superconducting joints. This generally leads to multiple interfaces between cooled component and superconducting joint, as may be seen in some of the examples described in GB 2453734.
Another approach, in which a superconductor joint is formed in thermal contact with a cooled component, but separated therefrom by an electrically isolating layer, is described in co-pending United Kingdom patent application No. GB1011475.9.
That document proposes improved superconducting joints and improved methods for forming superconducting joints in which only a single electrically isolating coating is positioned between the superconducting joint and the cooled component. The electrically isolating coating may be thinner, and is more thermally conductive, than the electrically isolating layers previously employed.